Molecular sieve materials have been used as adsorbers in adsorption beds through which a feed gas, such as air, is passed and to which various components of the feed gas will adhere depending generally on the type of adsorber and other factors such as gas pressure and temperature. Complete separation of the feed gas constituent gas cannot be achieved in a single pass through the adsorption bed, however, and the adsorbed gas must be periodically purged or removed from the system to regenerate its adsorption capability since the adsorber becomes saturated in that it can only hold a finite, usually small amount of the adsorbed constituent gas. Bed saturation and regeneration occurs in the following manner. Feed gas under pressure is forced through the adsorption from an input end to an output end. Gas exiting the output end is at first enriched in the desired component. However, as the enriched gas continues to evolve from the output end, evidencing that one component gas is being adsorbed in the bed, a line or front, behind which the adsorber is saturated with the absorbed component, advances through the adsorber from the input end toward the output end. When the front reaches the output end the adsorber is saturated and can hold no more of the adsorbed gas and the adsorbed gas "breaks through" the output end. The quality of the evolved gas is thereafter degraded. The bed, however, can be regenerated by interrupting the feed gas flow, venting the bed to a lower pressure and counterflowing a small quantity of enriched gas through the bed from the output end to the input end. Optimally, at least two parallel adsorption beds are used in a system to evolve a gas enriched in a desired component. While one bed is alternately pressurized with feed gas to evolve the desired gas, the other bed is alternately regenerated by counterflowing a small portion of the evolved desired gas through the bed to a vent.